Many attempts have been made at creating waste disposal systems that eliminate or reduce the need to landfill municipal solid waste (“MSW”). Traditional approaches have included incineration and pyrolisis. Conventional incineration however is objectionable because the high burn temperatures in the presence of oxygen results in the formation of complex pollutants that are difficult and expensive to control. Furthermore, the vast majority of incinerated organic material is converted into undesirable carbon dioxide, which is implicated in global warming, ozone layer depletion, and the formation of volatile organic compounds. The incineration process also releases nitrogen oxides that contribute to smog problems in urban areas. The pyrolisis procedure involves the conversion of various materials into a glass like residue in an oxygen depleted, high temperature environment. However, the high temperature, depleted oxygen environment of pyrolisis creates some extremely toxic compounds. Furthermore, pyrolisis is an inefficient method for disposing large volumes of waste materials, and the residual ash material contains large amounts of carbon.
Many of the disadvantages of incineration and pyrolisis are overcome by waste gasification. Waste gasification involves supplying the minimum amount of oxygen necessary to cause a thermo-chemical reaction that releases simple combustible gases at a controlled temperature, without supplying enough oxygen to cause combustion. When feed stock materials, such as MSW, that are rich in energy as measured by British thermal units, are loaded into gasification reactor chambers, and are exposed to a controlled temperature, oxygen depleted environment, such solid, sludge, or liquid feed stock materials are converted into a heavy vapor gas fuel. Materials that are rich in energy include, but are not limited to, coal, wood, cardboard, paper, industrial scrap, plastics, tires, organic wastes, sewage cake, animal waste, and crop residue, or a combination thereof. The released heavy vapor fuel gas is then mixed with oxygen and burned. Examples of prior gasification systems are shown in U.S. Pat. No. 4,941,415, which is incorporated herein by reference, and U.S. Pat. No. 5,941,184.
The material remaining after the completion of the gasification process cycle is composed of incombustible materials, including metals, glass, and ceramics, along with a fine inert salt and mineral power residue, and has a greatly reduced volume that is suitable for remanufacturing into concrete material or land filling. Furthermore, recyclable materials that do not undergo phase transition, such as all recycle glass, aluminum, metals, residual materials and salts, are recoverable after the gasification process, thereby eliminating the need for pre-sorting or processing the in-bound feed stock material.
Conventional prior art gasification systems are multi-step processes that generally utilize four open-looped process steps. These four steps typically involve: one or more primary gasifiers; a central air mixing chamber; a secondary processor for combusting the produced heavy vapor gas fuel; and final air cleaning systems. However, conventional gasification systems have proved difficult to cost-effectively construct. Therefore, a need exists for a simplified gasification apparatus that is inexpensive to build, simple to operate, and yet achieves the benefits of producing a gas fuel from solid waste feed stock materials.
Furthermore, prior art open-looped systems, such as U.S. Pat. No. 6,439,135, which is incorporated herein by reference, utilize exhaust stacks that release hot gases from the final combustion step into the atmosphere, or use storage tanks to collect the hot gases for future ancillary purposes, rather than reclaiming at least a portion of the cleaned air for re-introduction into the gasification process. Furthermore, such prior art systems do not teach a gasification system that produces a relatively pure carbon dioxide for other industrial purposes or to support the augmentation of vegetation, such as a greenhouse, a carbon dioxide dispersal system, or an aquaculture bed.
Current research indicates that increasing the surface area of the feed stock material that is exposed to gasification process gas significantly improves the production rate of fuel gas from the feed stock materials. Yet, prior art gasification systems, such as those illustrated in U.S. Pat. Nos. 6,439,135 and 5,619,938, utilize gasification reactor chamber configurations that expose only limited feed stock surface area to gasification process gas. Such prior art systems incorporate gasification reactor chamber configurations where only the bottom of the feed stock at grate level, known as the primary reaction zone, and the uppermost surface of the feed stock, known as the secondary reaction zone, are exposed to optimum gasification conditions.
As a result, gasification of the tons of feed stock material that is not located at either the primary or secondary zones, such as that on the sides and center of the gasification reactor chamber, requires that the temperature and duration of the gasification cycle be increased. Yet, higher gasification temperatures tend to reduce the Btu content of the resulting heavy vapor fuel gas. The high operating temperatures also increase the time required for cooling the gasification reactor chamber to a temperature suitable for the loading and disposal of subsequent loads of feed stock materials.
Furthermore, the costs associated with obtaining and maintaining the higher gasification temperatures, along with the cost of fabricating a complex gasification reactor chamber that can withstand prolonged exposure to high temperatures, also increase. Current gasification reactor chambers are lined with various clay-based insulative/refractory materials. These refractory materials maintain gasification reactor temperatures while also preventing structural damage to the gasification reactor chamber's steel superstructure and surface paint associated with prolonged exposure to excessive heat. Refractory material is usually applied to the gasification reactor chamber as pre-cast panels, bricks, or sprayed on as a gunnite-like application. Such refractory material is affixed to the exterior steel jacket of the gasification reactor chamber by refractory hangers, which are heavy metal dowels in the form of hooks. With typical prior art systems, a two to four inch layer of ceramic fiber blanket is usually inserted between the refractory material and the steel jacket before the refractory layer is installed to offer additional thermal protection for the exterior steel surfaces of the gasification reactor chamber.
Application of refractory material is thus labor intensive, time consuming, and a significantly expensive step. Additionally, the weight of the refractory liner necessitates that the steel vessel be constructed from at least ¼ inch thick hot rolled A36 steel plate and heavy structurals. This additional superstructure weight further increases the overall cost of manufacturing, shipping, and installation.
An additional problem with the use of refractory material is the length of time required for cooling the gasification reactor chamber before it can be re-used to gasify a subsequent load of MSW. More specifically, a subsequent gasification process typically cannot begin until the gasification reactor chamber has cooled to approximately 150 degrees Fahrenheit. Yet, at the end of a process cycle, the clay refractory material tends to retain heat for a long period of time. Depending on the particular chemistry of the refractory material, this retention of heat may require that the gasification reactor chamber be inoperative for several hours as the temperature of the chamber, and associated refractory material, cools down.
The limited feed stock capacity of prior art gasification systems often required the construction of multiple gasification reactor chambers to meet demand requirements. In previous designs, gasification reactor chambers typically have a rectangular configuration. As the length of the rectangular sidewalls is increased to satisfy larger feed stock capacity requirements, the size of the gasification reactor chamber creates problems associated with providing sufficient clearance space away from the prolonged high temperatures of the gasification reactor chamber. This problem typically limits gasification reactor chambers to configurations that are approximately 20 feet high, 20 feet wide, and 20 feet long. Such a configuration however has a limited load capacity of approximately 50 tons of feed stock material. Furthermore, as the size of the rectangular configuration is increased, problems develop with the side load waste dump arrangement. More specifically, as the rectangular sidewalls extend beyond 20 feet, the angle of repose of the trash spilling out of the garbage truck typically only fills a small portion of the gasification reactor chamber's near sidewall.
Because the heavy vapor fuel gas has been produced in an environment that typically contains no more than 8% oxygen, waste gasification systems must also increase the level of ambient oxygen in the gas produced in the gasification reactor chamber to make it fully flammable. This often requires increasing the oxygen content of the heavy vapor fuel gas to approximately 15% to 20%.
Prior art gasification systems increased the oxygen content of the heavy vapor fuel gas by directing the heavy vapor fuel gas through air mixing chambers. These mixing chambers are typically large, cylindrical vessels, with a variety of air induction tubes attached to multiple blower fans that flood the air mixing chambers with outside air using air compressors or high velocity fans. Yet, because of the large size of these chambers, they require substantial fabrication and installation time, and as a result are expensive. The use of fans and/or air compressors also increases the initial cost of the system and operating and maintenance expenses.
Conventional gasification systems also use cumbersome techniques for moving fuel gas to the point of combustion. Such systems often vent, or breech, the fuel gas from the top or at least one side of the gasification reactor chamber, and direct the vented fuel gas from the gasification reactor chamber into a secondary gas processor, which is usually driven by a natural draft current that is created by hot air in the system rising through an exhaust stack. The fuel gas' exit from the gasification reactor chamber is controlled by a motor driven damper assembly that regulates the varying flow of produced fuel gas from this first process step into ducting that connects the gasification reactor chamber to the secondary air mixing chamber. Such systems typically require large diameter piping to draw the gas off from the gasification reactor chamber. This large piping, and associated ductwork, increases not only equipment cost, but also installation expenses.
A further disadvantage of traditional air draft systems is that heavy vapor fuel gases have a tendency to linger in the gasification reactor chamber, and become subject to accidental combustion, which ultimately lowers the Btu content of the extracted heavy vapor fuel gas. This problem is exacerbated by the inconsistency of up-draft air movement in a natural draft system. Humidity, wind, barometric pressure and outside temperature all affect the rate of flow through a natural draft system. This inconsistent flow causes the evacuation of gases from the gasification reactor chambers to frequently stall, produces negative results in the process, and adversely effects the total cycle time for the gasification of the feed stock material.
Furthermore, the combustion of the heavy vapor fuel gas in a hot water heater, steam boiler, refrigeration unit, or other industrial process, produces a relatively high temperature exhaust. Yet, prior art systems often vent this hot combusted exhaust into the atmosphere at a temperature between 1200 and 1600 degrees Fahrenheit, thereby wasting a significant thermal resource that could be further captured and directly utilized in other heat dependent applications, thereby preserving natural resources and providing a cost efficient source for heated gas.
Hot combusted exhaust that is vented into the atmosphere in prior art systems via an exhaust stack also often contain large quantities of carbon dioxide. While carbon dioxide is not currently regulated as a pollutant from solid waste incinerators, it is subject to various industrial air quality abatement initiatives.
Furthermore, by recapturing the thermal energy that is entrained in the exhaust for additional attached applications, and thereby continuing to reduce the ultimate exhaust temperature of the exhaust gas, the volume of the exhaust decreases. As the volume of the exhaust gas is reduced, the size and quantity of conveying piping and other gas handling equipment, along with associated equipment costs, also decrease.
It is therefore an object of the present invention to provide a gasification system capable of gasifying feed stock at a reduced temperature and time.
It is another object of the present invention to decrease the time between subsequent uses of the gasification reactor chamber.
It is a further object of the present invention to provide a gasification system that produces a high Btu content vapor gas.
It is another object of the present invention to provide an inexpensive to build, simple to operate, gasification system that provides the benefits of producing a fuel gas from feed stock material.
It is another object of the present invention to provide for improved gas collection that allows for both simpler gasification reactor chamber configurations and an improved gas flow design that allows for better final combustion.
It is another object of the present invention to provide a gasification system that eliminates the need to rely on multiple gasification reactor chambers to provide an increased system volume capacity.
It is a further objective of the present invention to capture and sequester carbon dioxide produced by the gasification system, and to use the sequestered carbon dioxide in a beneficial manner.
It is another objective of the present invention to improve the quality of the final exhaust air from the present invention sufficiently to re-introduce the recycled process gas into the gasification system, thereby creating a closed-loop system.
These and other desirable characteristics of the present invention will become apparent in view of the present specification, including the claims and drawings.